Epicyclic gearboxes are frequently used in gas turbine engines for their compact designs and efficient high gear reduction capabilities. Planetary and star gear trains are well known, and are generally comprised of three gear train elements: a central sun gear, an outer ring gear with internal gear teeth, and a plurality of planet gears supported by a planet carrier between and in meshing engagement with both the sun gear and the ring gear. All three gear train elements share a common longitudinal central axis, about which at least two of them rotate. An advantage of epicyclic gear trains is their versatility. A rotary input can be connected to any one of the three elements. Holding one of the remaining two elements stationary with respect to the other two, permits the third to serve as an output.
In gas turbine engine applications, where a speed reduction transmission is required, the central sun gear generally provides rotary input from the powerplant. In planetary gear trains, the outer ring gear is generally held stationary and the planet gear carrier therefore rotates in the same direction as the sun gear and provides torque output at a reduced rotational speed. In star gear trains, the gear carrier is held stationary and the output shaft is driven by the ring gear in a direction opposite that of the sun gear.
However, certain shortcomings do exist with epicyclic drive trains. For example, as with many mechanical elements that transfer torque, a small but nevertheless significant amount of torsional deflection commonly occurs due to the elasticity of the material of the carrier, as a result of twist between upstream and downstream plates of the gear carrier, when the gear train is under load. The gear carrier twists around its central axis, causing the individual axis of rotation of the gears to lose parallelism with the central axis, of the gear carrier. This torsional deflection results in misalignment at gear train journal bearings and at the gear teeth mesh, which leads to efficiency losses and reduced life of the parts. Additionally, increased oil flow is required to the journal bearings to compensate for the misalignments caused by torsional deflections of the gear carrier plates.
Attempts to address this problem of planetary carrier torsional deflection are known. U.S. Pat. No. 5,466,198 issued Nov. 14, 1995 to McKibbin et al, for example, clearly sets out the problem and proposes a planetary gear train drive system which isolates the planetary carrier from torsional deflections. A torque frame or torque transfer structure is connected to a rotating load, such as a bladed propulsor. Pivotal joints, circumferentially disposed with respect to the carrier, each pivotable about a radial axis, connect axially extending arms of a torque frame to the planetary carrier. The pivotal joints, which are vital to the invention of McKibbin et al, permit the planetary carrier to be isolated from torsional deflections. While McKibbin et al do provide a device that eliminates planetary carrier torsional deflections, the planetary carrier system disclosed is of significant complexity. Both a low number of parts and low weight are characteristics vital in aircraft applications. Also, added parts, especially involving pivotable joints, increases the possibility of reliability problems.
Therefore, there remains a need for a simple, compact, device capable of transferring torque while eliminating twist within a planetary carrier.